Kinetic Study of Human β-1, 4-Galactosyltransferase Expressed in E. coli

Abstract

Recombinant β-1, 4-galactosyltransferase which synthesizes the Gal β1→4GlcNAc group of glycoprotein sugar chains was obtained as a soluble form from Escherichia coli by transfection of the human cDNA lacking the transmembrane segment. Kinetic study revealed that the soluble transferase has the same apparent K_m, values toward sugar nucleotide and sugar acceptors as those of mouse membrane-bound β-1, 4-galactosyltransferase previously characterized [Nakazawa et al. (1991) Eur. J. Biochem. 196, 363-368]. However, the V_max value of this transferase was low when compared to that of the mammalian transferase, probably due to the instability of the transferase caused by the lack of protein glycosylation. The soluble transferase was purified from the E. coli lysates almost to homogeneity by chromatography on DEAE-Sepharose and α-lactalbumin-Sepharose columns. Using this purified transferase, the acceptor specificity of the transferase has been studied. The results showed that the transferase has apparent K_m values of 170, 190, and 830 μM for agalacto-poly-N-acetyllactosamine, lacto-N-triose II, and lacto-N-triaosylceramide, respectively, but has apparently no activity toward glucosylceramide. These results suggest that the β-1, 4-galactosyltransferase may be involved in the synthesis of poly-N-acetyllactosamine, lacto-N-neotetraose, and probably lacto-N-neotetraosylceramide in addition to the formation of the Gal β1→4GlcNAc group of glycoprotein sugar chains and lactose.